Touch probe

ABSTRACT

A touch trigger probe for a coordinate measuring machine or machine tool has a stylus, which is biased into a rest position. The stylus has two independent supports, provided within a housing. A first support comprises a skirt of a stylus holder which is axially constrained on a surface of the housing. The second support comprises a kinematic arrangement of cylinders and balls, together with a planar spring, which together provide lateral constraint. The first support is biased by a much lighter force than the second support, so that its friction is very low. This reduces the lobing and hystersis of the stylus movement, and thereby increases the accuracy of the probe. The stylus movement first takes up a clearance between the first support and an abutment surface of the housing and then disengages the kinematic arrangement of the second support upon further stylus movement.

This is a continuation of application Ser. No. 08/426,733 filed Apr. 21,1995 now U.S. Pat. No. 5,491,904, which in turn is a continuation ofapplication Ser. No. 08/299,588 filed Sep. 1, 1994 now abandoned, whichin turn is a continuation of application Ser. No. 08/111,601 filed Aug.25, 1993 now U.S. Pat. No. 5,353,514, which in turn is a continuation ofapplication Ser. No. 07/944,341 filed Sep. 14, 1992 now U.S. Pat. No.5,253,428, which in turn is a continuation of application Ser. No.07/660,048 filed Feb. 25, 1991 now U.S. Pat. No. 5,146,691.

FIELD OF THE INVENTION

This invention relates to touch probes for position determiningapparatus such as machine tools and coordinate measuring machines.

DESCRIPTION OF PRIOR ART

Such machines have a spindle, quill or arm which is movable in two orthree dimensions and which can carry a touch probe or other tool. Scalesor other measuring devices measure the position of the probe or tool inthe two or three dimensions. A touch probe for use with such a machinehas a deflectable stylus extending from its body, for contacting aworkpiece surface.

Touch probes for such applications which have been extremely successfulcommercially, are shown in, for example, U.S. Pat. No. 4,153,998(McMurtry). Within the probe body there is a kinematic supportarrangement, into which a stylus holding member is urged axially by aspring. The kinematic support arrangement thus supports the stylus in arest position relative to the body, the rest position being extremelyrepeatable in space. When the stylus contacts a workpiece surface, atrigger signal is generated by the probe, which is used to trigger thetaking of a reading of the instantaneous position of the movablespindle, quill or arm. Because of the repeatability of the stylus restposition, this is an accurate measure of the position in space of thepoint of contact between the stylus and the workpiece. The stylus isdeflectable against the spring bias, to prevent damage, and subsequentlyreturns to the repeatable rest position.

Several different types of kinematic support arrangements are disclosedin U.S. Pat. No. 4,153,998. One arrangement disclosed has three supportlocations spaced around the axis of the probe. At each support location,there is a support element (e.g. a cylinder) on the stylus holdingmember, which is urged into engagement with a pair of support elements(e.g. a pair of balls) which have mutually convergent surfaces. Thisarrangement of three elements located in respective pairs of convergentsurfaces provides both an axial constraint and a lateral constraint onthe stylus when it is in the rest position, thereby providing a uniquekinematic rest position which is extremely repeatable.

A second alternative arrangement as described in U.S. Pat. No. 4,153,998has three support elements on one of the members, bearing against a flatsurface in the axial direction in order to provide an axial restposition. A separate lateral constraint is provided in order to give, incombination with the axial constraint, a unique kinematic rest position.Several different forms of lateral constraint are disclosed. Anotherexample of a lateral constraint which can be used is a planar springextending in the lateral direction, connected at one end to the stylusmember and at the other end to the housing. An example is shown in U.S.Pat. No. 4,451,987 (Cusack).

Although such probes have been widely used with great success, themeasurement requirements of industry are nowadays becoming moredemanding, and so it is becoming increasingly necessary to take accountof certain problems of such probes which were previously of lessimportance.

For example, because of the three support locations, such probes exhibita phenomenon known as "lobing". This can be explained as follows. As theprobe travels towards a workpiece surface, there is always a smalldistance travelled between the instant at which the stylus contacts theworkpiece, and the instant at which the probe generates a trigger signalas a result of such contact. This is known as "pre-travel". For a givendirection of contact with the workpiece, the pre-travel is extremelyrepeatable, and its effects can therefore be compensated by calibration.However, because of the nature of the three support locations, thepre-travel varies considerably for different directions of contact,giving a lobed characteristic to the response of the probe in a planenormal to the axis, (known as the XY plane) and necessitating adifferent calibration and compensation value for each direction. Thelobing effect is proportional to the axial force which urges thekinematic seating elements together, especially in the case where thetrigger signal is generated by electrical switching at the seatingelements.

To reduce the lobing effect, it is obviously desirable to have as low aspring force as possible. However, there is a practical lower limit tothe spring force, since a certain minimum spring force is required tokeep the kinematic support elements seated on each other in the event ofacceleration or vibration of the probe as it is moved; otherwiseconsequent unseating will result in false trigger signals beinggenerated.

Such convergent surface probes can very occasionally also suffer from aproblem of hysteresis when reseating. This can be caused by frictionbetween the seating elements, causing the single element at one of thelocations not to seat fully with the corresponding pair of convergentsurfaces. The consequent vertical error in the position of this element,causing tilting of the stylus member, gives rise to a correspondinglateral error in the rest position of the stylus tip. The direction ofthis lateral error depends on the direction from which the stylus isreseating. The lateral error is magnified by a lever effect proportionalto the length of the stylus, and is rather more significant than anylateral reseating error of the support element in its pair of convergentsurfaces.

The other type of probe noted above, having three support elementsresting on flat surfaces, and a separate lateral constraint, also hasthe disadvantages of lobing and hysteresis. Although the stylus memberreturns to a repeatable rest position for a given direction of contactwith the workpiece, different directions of contact in the XY plane tendto give slightly different rest positions. This is also due to frictionbetween the support elements and the supporting flat surface, as theyslide over each other in the XY direction. As the probe reseats, thesupport elements slide over the flat surface in the XY plane, until thetension of the lateral constraint acting in the XY plane reduces to acertain residual level at which it is unable to overcome the remainingfrictional force in the XY plane at the support element surfaces.

Probes are also known having an axial constraint consisting of more thanthree support elements upon a flat surface (e.g. U.S. Pat. No.4,763,421), or of a continuous annular ring supporting or supported on aflat surface (e.g. U.S. Pat. No. 4,477,976). Whilst this may reduce thelobing problem, the hysteresis problem remains and may even be worse.

SUMMARY OF THE INVENTION

Preferred embodiments of the present invention incorporate advantageousfeatures of both types of prior art probe discussed above, whileminimising their respective disadvantages.

In a preferred embodiment, a touch probe comprises a fixed member, afirst movable member for carrying a workpiece contacting stylus, firstsupport means comprising at least one pair of first mutually engageableelements, and second support contacting stylus, first support meanscomprising at least one pair of first mutually engageable elements, andsecond support means, independent from the first support means,comprising at least one pair of second mutually engageable elements. Thetwo support means co-operate with each other when all said elements areengaged to hold the first movable member in a rest position relative tothe fixed member.

The touch probe also includes first and second bias means for biasingall said elements into engagement. Both said pairs of elements aredisengageable against the action of the bias means to permitdisplacement, in the same direction, of said first movable memberrelative to the fixed member out of said rest position when said styluscontacts a workpiece. The pair of first elements is biased intoengagement with a lower force than said pair of second elements, wherebyupon displacement of the first movable member, said pair of firstelements disengages, against said lower bias force, and thensubsequently said pair of second elements disengages.

The touch probe further includes means for providing a signal when saidstylus contacts a workpiece.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred embodiments will now be described with reference to theaccompanying drawings, wherein:

FIG. 1 is a vertical cross section through a first probe,

FIG. 2 is a horizontal cross section on the line II--II in FIG. 1,

FIG. 3 shows a detector arrangement for the probe of FIG. 1,

FIG. 4 is a view corresponding to FIG. 2, but showing a modification,

FIG. 5 is a vertical cross section through a second probe,

FIG. 6 is a horizontal cross section on the line VI--VI in FIG. 5,

FIG. 7 is a vertical cross section through a third probe,

FIG. 8 is a view on the line VIII in FIG. 1, showing anothermodification,

FIG. 9 is a view corresponding to FIG. 8, showing a still furthermodification; and

FIG. 10 is a section through a modification of the probe in FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring firstly to FIGS. 1 and 2, the probe shown comprises a housing10, within which is located a stylus holder 12. The stylus holder 12carries a stylus 14, which projects through an aperture 16 in thehousing 10. The stylus has a tip 15 for contacting a workpiece, and suchcontact causes deflection of the stylus.

The stylus holder 12 is cup-shaped, having a depending skirt 18. Theannular bottom edge 22 of the skirt 18 rests on a flat internal surface20 of the housing 10. The surfaces 22 and 20 are manufactured by lappingthem together, so that they are extremely flat and conform with eachother. This ensures that the stylus holder 12 is supported in a preciseaxial rest position. It is biased into this axial rest position by aspring 24, which causes a conical member 26 to bear in a recess 28 inthe stylus holder.

As well as being located in a precise axial rest position, the stylusholder 12 and its depending stylus 14 are also located in a very preciselateral rest position. This is ensured as follows. A diaphragm-likeplanar spring 30 is secured in its central region to the stylus holder12, at the point from which the stylus 14 depends from the stylus holder12. At its outer perimeter, the planar spring 30 is secured to a ring32. Three cylinders 34 project radially from the ring 32, and seat inthe convergent surfaces provided by respective pairs of balls 36, whichare fixed on the housing 10. Thus, the ring 32 is located in akinematically-defined rest position, both axially and laterally, in thesame manner as described in U.S. Pat. No. 4,153,998. With respect to thering 32, the planar spring 30 holds the stylus holder 12 in an extremelywell-defined lateral rest position. However, the planar spring 30 canflex to allow vertical movement and tilting of the stylus holder 12 withrespect to the ring 32; such movements are only constrained by the axialrest position provided by the surfaces 20 and 22.

Thus, the stylus 14 is held in a precisely defined rest position by acombination of an axial constraint and a lateral constraint. Althoughthe balls 36 and cylinders 34 provide axial constraint as well aslateral constraint, the axial flexibility of the planar spring 30ensures that the stylus holder 12 is not over-constrained.

A dowel 38 is fixed to the housing 10 and extends through a hole 40 inthe stylus holder 12, with sufficient clearance so as not to interferewith normal operation of the probe. The purpose of this dowel is toprevent rotation of the stylus holder 12 when an exchangeable stylus 14is being screwed into the stylus holder. The dowel also provides ameasure of guidance to ensure that the cylinders 34 correctly reseat inthe pairs of balls 36 after a deflection of the stylus.

The planar spring 30 is pre-stressed, and in its relaxed state it isbowed upward, in the manner of a Belleville washer. However, it assumesthe flat, stressed condition shown in FIG. 1 under the biasing providedby the spring 24. Considering the forces acting upon the stylus holder12 in the rest position, the downward force provided by the spring 24 isonly slightly greater than, and is largely counterbalanced by, theupward bowing force of the planar spring 30. Consequently, the pressurebetween the surface 20 of the housing and the surface 22 of the skirt 18is very small. Since the friction between the surfaces 20 and 22 isproportional to this pressure, the friction is also very small.

When the stylus 14 contacts a workpiece, from any direction, the stylusis deflected. For example, if the contact is in a horizontal direction,the stylus 14 tilts, about a point of contact between the surfaces 20and 22. At this time, the cylinders 34 and balls 36 remain engaged witheach other, and the tilting is accommodated by flexing of the planarspring 30. As deflection of the stylus 14 continues, and theprestressing of the planar spring 30 is relieved, the further deflectionis accommodated by lifting of at least one of the cylinders 34 out ofits seating with the associated balls 36.

Because the initial deflection is accommodated only by the tilting ofthe skirt 18 on the surface 20, and not by movement of thekinematically-mounted ring 32, the force required to produce thedeflection is the same from all horizontal directions. This is becauseof the annular nature of the skirt 18. As a result, this initialdeflection is entirely non-lobing. It is also extremely sensitive,because of the low contact force between the surfaces 20 and 22.

When the deflecting force on the stylus 14 ceases (i.e. when the probeis moved so that the stylus 14 no longer contacts the workpiece) thestylus member 12 is returned to its axial and lateral rest position bythe action of the spring 24. In doing so, it is likely that there willbe some rubbing of the surfaces 20 and 22 against each other, and alsoof the cylinders 34 against the balls 36. The friction inherent in suchrubbing is a source of hysteresis in conventional probes, when thespring force is unable completely to overcome the frictional forces toreturn the stylus holder to the precise rest position. In the presentprobe this is reduced because of the very light pressure between thesurfaces 20 and 22, which means that the frictional forces are extremelylow. Thus, there is little frictional force preventing the skirt 18moving laterally to the lateral rest position; and since the surfaces 20and 22 are flat, there is no resistance preventing the skirt 18 frommoving into the axial rest position.

During the reseat operation, the lateral rest position is in factassured positively by the cylinders 34 and balls 36, and by the planarspring 30. It is of course still possible for frictional forces betweenthe cylinders 34 and balls 36 to have an effect. Thus, friction betweenthe cylinders 34 and balls 36 may prevent one or more of the cylinders34 from completely reseating in its rest position between thecorresponding pairs of balls 36. However, any resulting mis-positioningwill largely be in the vertical direction rather than the horizontaldirection. This is accommodated by a flexure of the planar spring 30,and therefore does not disturb the axial rest position of the stylusholder 12 (since this is dependent upon the skirt 18 returning to itsaxial rest position). Such mis-positioning between the cylinders 34 andballs 36 has a much less significant effect upon the lateral restposition. Furthermore, any lateral mis-positioning which might occurcauses only a lateral translation of the rest position of the stylus 14;it does not cause it to tilt as in the case of the prior art. Thus, themagnitude of the mis-positioning of the stylus tip 15 is only the sameas the magnitude of the lateral mis-positioning of the ring 32, whereasin the prior art probes the leverage effect caused by the length of thestylus 14 would greatly magnify the mis-positioning of the stylus tip 15when the stylus 14 tilts.

FIG. 8 shows a modification which can reduce any lateral mis-positioningof the ring 32 even further. In place of the balls 36, each roller 34 islocated between two cylindrical rollers 36a, forming a V groove. Thesides of the V groove are deliberately made very steep, so that anymismatch is more evident in the vertical direction than in the lateraldirection. If desired, a similar effect could be achieved with the balls36 of FIG. 1 by setting them slightly further apart than would beconventional.

FIG. 9 shows a further modification of the type shown in FIG. 8. Itcomprises two cylinders 106, in place of the pair of balls 36. Thecylinders 106 are seen end-on in FIG. 9, and are parallel to each other,lying radially with respect to the axis of the probe. Between them,there is a recess 108 in the housing 10. In place of the cylinder 34 inFIG. 1, a conical element 110 depends from the ring 32. In the kinematicrest position, this conical member 110 is urged against the pair ofcylinders 106 as shown in the figure. The apex of the conical element110 has a cylindrical extension 112, which in the rest position isaccommodated in the recess 108.

The cone angle of the conical element 110 is made very steep. Inconsequence, any mis-positioning due to friction effects when the ring32 is brought back to the rest position, occurs primarily in thevertical direction rather than in the lateral direction. The steepnessof the cone angle thus performs the same function as the steep sided Vformed by the cylinders 36a in FIG. 8. The extension 112 remains betweenthe cylinders 106 when the ring 32 lifts and the conical element 110disengages. This extension 112 thus acts as a guide to ensure correctreturn of the conical element 110 into contact with the cylinders 106,and it also obviates the need for the dowel 38.

The instant at which the stylus tip 15 first contacts a workpiece can bedetected in various possible ways. For example, it is possible to mountpiezo-electric detectors or strain gauges on the stylus holder 12 todetect the initial contact (for example as described in InternationalPatent Specification No. WO86/03829). Alternatively, the contact can bedetected by arranging that an electric circuit between the stylus tip 15and the workpiece is closed by the contact. As shown in FIG. 1, however,our presently preferred method is photo-electric. A light emitting diode42 produces a beam of light which passes through an opening 44 in thestylus holder 12. This produces a spot of light 48 upon a pair of lightdetectors 46 arranged side by side as seen more clearly in FIG. 3. Theoutputs of the detectors 46 are taken to an auto zero circuit 50 havingdifferential inputs. The function of the circuit 50 is to track anylong-term drift of the outputs of the detectors 46, so that the outputof the circuit 50 is a constant value (e.g. zero) despite any suchlong-term drift. However, if there is any more sudden mismatch betweenthe differential inputs received from the detectors 46, the auto zerocircuit 50 reacts, its output increases, and a trigger circuit 52produces a trigger signal as a result. Thus, the auto zero circuit 50 isresponsive to rates of change of the differential inputs which exceed agiven value, such as would be caused when the light spot 48 movestowards one or other of the detectors 46 when the stylus 14 begins todeflect upon contact with a workpiece. Further description of such anauto zero circuit can be found in EP No. 0242747.

FIG. 4 shows a modification of the probe of FIGS. 1 and 2. The planarspring 30, ring 32 and cylinders 34 are replaced by a generallytriangular planar spring 54. At each of the apices of the triangle, aradial slot 56 is cut into the planar spring. This defines a fork ateach apex. In place of the three pairs of balls 36 fixed to the housing10, there are three single balls 58, and each slot 56 locates over oneof the balls 58. This arrangement provides a precise lateral restposition in the same manner as FIG. 1, but is less expensive tomanufacture.

The skirt 18 in FIG. 1 may be made from a ceramic material, and may bearagainst a ceramic annulus in the housing 10 forming the surface 20. Theceramic materials are preferred because they can be lapped against eachother to produce a seating having a very high degree of flatness, andthus precision.

Alternatively, as shown in FIG. 10, in place of the lapped surfaces 20and 22, the skirt 18 may have on its undersurface three balls 114,equispaced around the axis of the probe, which bear against the surface20. As described in U.S. Pat. No. 4,153,998, such an arrangement ofthree balls 114 on a plate gives a very precise axial rest position,without the need for lapping. However, because of the three pointsupport, such an arrangement is subject to some degree of lobing. Whilethe lobing is worse than the arrangement in FIG. 1, it is neverthelessbetter than the prior art, because of the very low force between theskirt 18 and the surface 20. The degree of lobing, of course, isproportional to this force, which must be overcome in order to deflectthe stylus.

If such an arrangement of three balls 114 is used, it becomes possibleto detect the deflection of the stylus by electrical switching betweenthe balls 114 and the surface 20. This is described more fully in U.S.Pat. No. 4,153,998. In such a case, it is possible for the balls 114 onthe skirt 18 to be much smaller than would be necessary in the priorart. The prior art arrangement needs balls of a relatively large radiusin order to give the loading capability. However, with the probe of FIG.10, the loading between the skirt 18 and the surface 20 is very small,so that small balls 114 can be used. This ensures that the same Hertzianstressing occurs between the balls and the surface 20 as in the priorart large radius balls, so that the electrical characteristics of thecontacts are unaffected, whilst still keeping a low loading between theskirt 18 and the surface 20 so as to minimise lobing.

It is desirable to ensure that the respective weights of the stylusholder 12 and stylus 14 are chosen such that the centre of gravity CG ofthe stylus and stylus holder combination is roughly in the samehorizontal plane as the axial and lateral constraints which define therest position. This helps to ensure that, despite the relatively lowaxial loading which keeps the skirt 18 in place on the surface 20,nevertheless there is little risk of accidental deflection of the stylus14 owing to vibration or inertial effects as the probe is moved.

As an alternative to the arrangement in FIG. 10, a ring of balls may beprovided on the undersurface of the skirt 18 to bear against the surface20. Such a ring of balls is manufactured by inserting them in an annularplate, each in a respective bore, and bonding them in place withcapillary glue. The annular plate is clamped against the surface 20,with the balls in contact with the surface 20, while the glue sets. Thisensures that there is an accurate axial rest position, with all theballs contacting the surface 20, and gives a substantially non-lobingperformance.

FIGS. 5 and 6 show a second probe according to the present invention.Within a housing 60, there is a stylus holder 62, from which depends astylus 14 as previously. The stylus holder 62 lies within anintermediate cage 64, and is located within that cage by a kinematicarrangement of cylinders 66 and pairs of balls 68, arranged generallysimilar to the cylinders 34 and balls 36 of FIG. 1. The cylinders 66 andballs 68 are biased into engagement with each other by a spring 70within the cage 64.

The cage 64 has a depending annular skirt 72, which engages with asurface 74 of the housing 60, in the same way as the skirt 18 engageswith the surface 20 of the housing 10 in FIG. 1. As in FIG. 1, the skirt72 and the surface 74 may be of ceramic, and are manufactured by lappingagainst each other so as to give a good axial rest position for the cage64. The cage 64 is biased into this rest position by a spring 76. Theforce on the cage due to the spring 76 is relatively small, and muchsmaller than the force on stylus holder 62 due to the spring 70, so thatthe axial load between the skirt 72 and the surface 74 is very small.This gives very low friction between the skirt 72 and the surface 74,which gives good lobing and hysteresis characteristics in the same wayas the probe of FIG. 1.

The cage 64 is located laterally in a rest position by a planar springarrangement. As can be seen in FIG. 5, this is connected directlybetween the cage 64 and the housing 60, and unlike FIG. 1 is notdirectly secured to the stylus holder 62. One arrangement provides threeelongate planar springs 78, in a triangular arrangement grouped aboutthe axis of the probe, as more clearly seen in FIG. 6. One end of eachspring 78 is secured to the cage 64, while the other end of each issecured to the housing 60. These springs 78 are not prestressed, but areflat in a relaxed condition, and thus do not contribute to the loadingbetween the skirt 72 and the surface 74.

Thus, the stylus holder 62 is supported in a completely kinematic mannerin the cage 64 by the cylinders 66 and balls 68, and is urged into thissupport by the spring 70; all of these elements inside the cage 64comprise a closed system which can move bodily with respect to thehousing 60. The cage 64, in turn, has a good axial constraint providedby the skirt 72 and surface 74; and a good lateral constraint providedby the planar springs 78.

In operation, when the stylus 14 is deflected by contact with aworkpiece, at first the skirt 72 and cage 64 lift or tilt bodily fromthe surfaces 74. The stylus holder 62 is prevented from lifting withinthe cage 64 by three permanent magnets 80 (only one of which can be seenin the figure), which retain the cylinders 66 in engagement with theballs 68. This bodily lifting or tilting of the cage 64 is opposed onlyby the small force of the spring 76, giving the low loading between theskirt 72 and the surface 74, so that the initial movement of the probehas a non-lobing characteristic, for the same reason as in the probe ofFIG. 1. Also for the same reason, when eventually the stylus returns toits rest position, there is little or no hysteresis in its restposition.

However, the above bodily lifting or tilting of the cage 64 upondeflection of the stylus only lasts for a very small amount of stylusdeflection. There is only a very small clearance 82 (e.g. 10 microns)between the upper edge of the skirt 72 and a shoulder 84 which extendsaround the inside of the housing 60. After sufficient deflection to takeup this clearance 82, further deflection of the stylus 14 causes one ormore of the cylinders 66 to disengage from the respective balls 68,against the action of the spring 70 and magnets 80.

In this embodiment of FIGS. 5 and 6, the annular lapped contact regionbetween the skirt 72 and the surface 74 may if desired be replaced bythree balls engaging against a plate, or by a ring of balls engagingagainst a plate, as described above in relation to the modification ofFIG. 1 shown in FIG. 10.

FIG. 7 shows a further embodiment. A cage 86 is provided within ahousing 88. The cage 86 is kinematically located on balls 90 and rollers92, similar to those numbered 34 and 36 in FIG. 1. A spring 94 biasesthe cage 86 into a kinematically-defined rest position on thesecylinders and balls.

Inside the cage 86, there is a stylus holder 96, from which depends astylus 14 as previously. The stylus holder 96 is biased into a restposition within the cage 86 by a spring 98 with a smaller force thanthat which biases the cage 86 into its rest position. The rest positionof the stylus holder 96 within the cage 86 is defined laterally by adiaphragm-like planar spring 100, which is connected to the stylusholder in its central region in the same manner as the planar spring 30in FIG. 1. However, the planar spring 100 is not prestressed, but is ina relaxed condition when flat as shown in FIG. 7. The rest position ofthe stylus holder 96 is axially defined by flat surfaces on the stylusholder 96 and cage 86 at 102. These surfaces are lapped with each otherto give good axial positioning, as previously. The respective componentsmay be of ceramic material to facilitate this as previously.

In operation, when the stylus 14 is deflected by contact with aworkpiece, the cage 86 initially remains stationary in its kinematicrest position defined by the balls and cylinders 90,92. The initialdeflection is taken up by movement of the stylus holder 96 within thecage 86, against the action of the spring 98, such movement beingpermitted by flexing of the planar spring 100. The axial loading betweenthe stylus holder 96 and the cage 86, at the surfaces 102, is verylight, as previously, by reason of the light spring 98. In consequence,the deflection of the stylus is non-lobing and substantiallyhysteresis-free, for the same reasons as previously.

Only a relatively small amount of movement of the stylus holder 96within the cage 86 is permitted, before the stylus holder 96 engages ashoulder 104 within the cage. As with the shoulder 84 in FIG. 5, thereis only a small clearance before this happens. Subsequent deflection ofthe stylus 14 is accommodated by bodily movement of the cage 86, out ofits kinematic seating elements 90,92, against the action of the spring94.

In a similar manner to the modification of FIGS. 5 and 6 discussedabove, the lapped flat surfaces 102 in FIG. 7 can be replaced by threeballs bearing on a plate, or a ring of balls bearing on a plate.

All of the embodiments of FIGS. 4-10 may have any of the arrangementsfor detecting the instant of contact between the stylus tip and aworkpiece, as discussed above in relation to the FIG. 1 embodiment. Thecentre of gravity of the stylus and stylus holder may also be asdiscussed in FIG. 1.

It is a feature of all the embodiments described that the planar spring30,78 or 100 does not have to flex for any great distance. In the caseof the probes of FIGS. 5 to 7, this is because the small clearancebefore engagement with the shoulder 84 or 104 prevents any furtherflexing of the planar spring. Further overtravel of the stylus 14 is ineach case accommodated by the disengagement of the cylinders and balls66,68 and 92,90, without further flexing of the planar spring. In thecase of the probe of FIG. 1, although there is no shoulder preventingflexing, nevertheless the overtravel of the stylus can be accommodatedby disengagement of the cylinders and balls 34,36 without flexing of theplanar spring 30. This is in contrast to prior art probes in whichlateral constraint is provided by a planar spring, (such as U.S. Pat.No. 4,451,987 referred to above) which must also permit overtravel. Suchprior art probes require a relatively floppy planar spring to allow forsuch stylus overtravel. With the probes described above, this is notnecessary, and so the planar spring can in each case be madesubstantially stiffer. This considerably improves the force with whichthe planar spring resists any residual lateral displacement of the skirt18 in FIG. 1, the skirt 72 in FIG. 5, or the stylus holder 96 in FIG. 7.The greater lateral resistance results in further reductions in thehysteresis of the frictional contact between the surfaces 20,22 (FIG.1); between the skirt 72 and surface 74 (FIG. 5); and between thesurfaces 102 (FIG. 7).

A further feature of all the probes described is that while the forcerequired for the initial deflection of the stylus is deliberately madevery low, to reduce or eliminate the lobing and hysteresis problems,nevertheless the force causing the cylinders 34,66,92 to seat in theball pairs 36,68,90 is still very high, so that good reseating can beassured. In fact, the force with which the cylinders are urged intoengagement with the respective balls can be higher than would normallybe the case with prior art probes having such kinematic seatingarrangements, since increasing this force does not reduce the lobing andhysteresis performance of the probe.

I claim:
 1. A touch probe, for use on a movable arm of a positiondetermining apparatus, the probe having a housing with an axis and astylus holder located within the housing, the stylus holder carrying anelongate stylus which projects through an aperture in the housing, andwhich has a sensing tip at a free end thereof, the probe generating atrigger signal when said sensing tip contacts an object and said stylusholder is thereby deflected relative to said housing, the trigger signalbeing used by the position determining apparatus to take a reading of aninstantaneous position of the movable arm, the touch probe comprising:aseating including at least one pair of mutually engageable elements, afirst mutually engageable element of the at least one pair of mutuallyengageable elements being connected to the housing, and a secondmutually engageable element of the at least one pair of mutuallyengageable elements being connected to the stylus holder, each of saidfirst and second mutually engageable elements having a surface inclinedrelative to the axis; at least one constraining spring connected to thehousing and to the stylus holder; biasing means for applying an axialbiasing force to said stylus holder for biasing said first and secondmutually engageable elements of said seating into engagement with eachother; an annular member retained in a predetermined relationship withthe stylus holder and having an annular surface facing in a direction ofsaid aperture, said stylus holder being tiltable with said annularmember relative to the housing on said annular surface; a transducerdistinct from said first and second mutually engageable elements of saidseating for generating said trigger signal, said transducer beingactuable by tilting of said stylus holder with said annular member; andsaid probe further comprising an abutment surface connected to thehousing and said annular member being separated from said abutmentsurface by a clearance, said stylus holder thereby having a firsttilting action relative to said housing before said clearance is takenup, and a second tilting action, distinct from said first tiltingaction, after said clearance is taken up, said first and second mutuallyengageable elements of said seating remaining engaged during said firsttilting action and coming out of contact with each other during saidsecond tilting action.
 2. A touch probe, for use on a movable arm of aposition determining apparatus, the probe having a housing with an axisand a stylus holder located within the housing, the stylus holdercarrying an elongate stylus which projects through an aperture in thehousing, and which has a sensing tip at a free end thereof, the probegenerating a trigger signal when said sensing tip contacts an object andsaid stylus holder is thereby deflected relative to said housing, thetrigger signal being used by the position determining apparatus to takea reading of an instantaneous position of the movable arm, the touchprobe comprising:a seating including at least one pair of mutuallyengageable elements, a first mutually engageable element of the at leastone pair of mutually engageable elements being connected to the housing,and a second mutually engageable element of the at least one pair ofmutually engageable elements being connected to the stylus holder, eachof said first and second mutually engageable elements having a surfaceinclined relative to the axis; biasing means for applying an axialbiasing force to said stylus holder for biasing said first and secondmutually engageable elements of said seating into engagement with eachother; an annular member retained in a predetermined relationship withthe stylus holder and having an annular surface facing in a direction ofsaid aperture, said stylus holder being tiltable with said annularmember relative to the housing on said annular surface; a transducerdistinct from said first and second mutually engageable elements of saidseating for generating said trigger signal, said transducer beingactuable by tilting of said stylus holder with said annular member; andsaid probe further comprising an abutment surface connected to thehousing and said annular member being separated from said abutmentsurface by a clearance, said stylus holder thereby having a firsttilting action relative to said housing before said clearance is takenup, and a second tilting action, distinct from said first tiltingaction, after said clearance is taken up, said first and second mutuallyengageable elements of said seating remaining engaged during said firsttilting action and coming out of contact with each other during saidsecond tilting action.
 3. A touch probe, for use on a movable arm of aposition determining apparatus, the probe having a housing with an axisand a stylus holder located within the housing, the stylus holdercarrying an elongate stylus which projects through an aperture in thehousing, and which has a sensing tip at a free end thereof, the probegenerating a trigger signal when said sensing tip contacts an object andsaid stylus holder is thereby deflected relative to said housing, thetrigger signal being used by the position determining apparatus to takea reading of an instantaneous position of the movable arm, the touchprobe comprising:a seating including at least one pair of mutuallyengageable elements, a first mutually engageable element of the at leastone pair of mutually engageable elements being connected to the housing,and a second mutually engageable element of the at least one pair ofmutually engageable elements being connected to the stylus holder, eachof said first and second mutually engageable elements having a surfaceinclined relative to the axis; at least one constraining springconnected to the housing and to the stylus holder; biasing means forapplying an axial biasing force to said stylus holder for biasing saidfirst and second mutually engageable elements of said seating intoengagement with each other; an annular member retained in apredetermined relationship with the stylus holder and having an annularsurface facing in a direction of said aperture, said stylus holder beingtiltable with said annular member relative to the housing on saidannular surface; a transducer for generating said trigger signal, saidtransducer being actuable by tilting of said stylus holder with saidannular member; and said probe further comprising an abutment surfaceconnected to the housing and said annular member being separated fromsaid abutment surface by a clearance, said stylus holder thereby havinga first tilting action relative to said housing before said clearance istaken up, and a second tilting action, distinct from said first tiltingaction, after said clearance is taken up, said first and second mutuallyengageable elements of said seating remaining engaged during said firsttilting action and coming out of contact with each other during saidsecond tilting action.